Quadrature amplitude modulation (QAM) can be used as an analog or digital modulation scheme, and is used extensively in many telecommunication systems. QAM conveys two analog message signals or two digital bit streams by changing (modulating) the amplitudes of two carrier waves, where in the carrier waves are 90° out of phase with each other and are usually sinusoids. After the two carrier waves are modulated, the resulting modulated waveforms (which are often referred to as an “I-channel” and “Q-channel”) are added together (summed). In the digital case, this summed waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK); while, in the analog case, the summed waveform is a combination of phase modulation (PM) and amplitude modulation (AM).
Regardless of whether a digital bit stream or analog message is to be conveyed, QAM transmitters and receivers include I/Q phase shifters to help generate the 90° phase shift for their I and Q channels. As the inventors have appreciated, conventional I/Q phase shifters are less than ideal for several reasons. For example, the loading impedance of an I/Q phase shifter (e.g., made up of a Gilbert cell) is dependent on temperature, input power, manufacturing process variations, frequency, and so on, such that the performance of the I/Q phase shifter can vary widely, leading to phase shifts that “wander” from 90° over time. Therefore, the inventors have devised improved low-loss, broad band, LC I/Q phase shifters as described further herein.